Issues with mosfet heat

I assembled the board and noticed that the mosfet was getting hot, when through everything and found a stray cut wire that was shorting two pins of the 4 pin connector. I fixed this, and the mosfet stayed relatively cool. In the mean time, I was waiting on some components to finish the wifi module assembly. Once I had everything together, I did not check the mosfet again, but it was functioning fully (with wifi). I let it run this way for less than an hour when I smelled smoke, the mosfet was very hot, and it had burned part of the board. I tried to remove the mosfet and of course also pulled up the trace between it and the 100mH inductor. I replaced the mosfet (IRF640N) (largely based on reading this thread: www.tubeclockdb.com/forum/12-arduino-nix...nixie-clock?start=12 )and was getting just a green LED and the power light to flash.

I figured the heat/power must have taken out the inductor as well, so I went ahead and replaced that with a slightly higher amperage and lower tolerance 100mH inductor. But I have the same problem. Only the first tube LED flashes green, and the power light flashes when I turn it on. No high voltage is running on the board. Not sure what else I could have damaged and how I should diagnose.

Some quick info: I hhave the modular arduino kit, I have a good multimeter, but I do not have a scope. I'm very much a "hobby" electronics person, with no real background in it. Thanks for any help you might have.

Hello.
I made 6-digit All-in-One clock with my own PCB design used Tesla ZM1082 nixies.

I have a problem with the temperature of the MOSFET (used IFR740, 100uH choke Coilcraft DR0810-104L, UF4007 diode, 2u2 / 450V Panasonic cap).
Calibration runs fine when finished automatically sets FET PWRon around 220-230. This seems to be too much, the transistor has about 100-120 degrees. If I manually reduce the PWRon value to 150, HV will drop to 145-150V and the temperature will drop to about 90 degrees. That's a lot, too.

Can it be caused by a high Rdson transistor (550mOhm) and a low coil impedance (220mOhm)? I'm going to make the following changes:
1. I increase the inductance of the coil to 330uH with an impedance of 1 Ohm
2. change the MOSFET to a smaller Rdson type ... about 200mOhm (SIHP25N40D or STP24N60DM2)

Do you think these changes could help? Which change should I make first?

I note that the clock works normally, the LEDs work, the RTC module also. Atmega works, the HVD signal has a nice course on the scope. But because of the high temperature, I don't want to keep them running for more than a few minutes.

Tahks for help ... Vladca

Hello.
In the local store, they only had the IRFP360 (Rdson 0.2 Ohm) in a TO247 package and a 330uH/0.5 Ohm coil. I bought them for tests.

I replaced the coil first, but the result was not quite good.
After replacing the MOSFET, great results came. HV will run at 180V much faster than before (a few seconds). It is held stably and the PWRontime calibration ended with a result of 178. And most importantly the temperature dropped to 32 degrees (measured using a contact PT100 probe). I can also use a 9V power supply without any problems.
Finally, I returned the original 100uH/0.2Ohm coil to see what it would do. It works. MOSFET temperature slightly higher, HV less stable and longer PWRontime.

I'll look for suitable replacements for the MOSFET and do other tests with 150-220uH coils, which will probably be better.

I'll write the results here, maybe it will help someone in the future.

Vladca